1. Field of the Invention
The invention relates to optical communication cables and somewhat more particularly to optical communication cable structures useful in pressurized environments.
2. Prior Art
Optical communication cable structures are known and in such cable structures, the cable sheath functions to protect optical leads therein, which for example are comprised of glass fibers. In certain known optical cable structures, a given plurality of optical leads or elements are loosely stranded to form bundles and such bundles are stranded with one another to form a cable core which is enveloped by a protective sheath consisting of an inner sheath, a metal foil encasing the inner sheath and an outer sheath about the metal foil. The metal foil forms a vapor-barrier for the cable core, which can be provided with a jelly-like lubricant (see German OS No. 2,302,662 which generally corresponds to U.S. Pat. No. 4,072,398). In another known cable structures, optical leads are stranded around a tension-releaving element and hollow spaces within the resultant cable core are filled with a petrolatum-like material and enveloped with a polyethylene sheath. The petrolatum-like material improves the friction ratio between the various elements of the cable structure and provides longitudinal moisture-impermeability (see German OS No. 2,429,670, which generally corresponds to British Patent Specification No. 1,455,732).
In order to further improve the mechanical protection provided by a cable sheath in optical cable structures, it is known to form an inner cable sheath of thermoplastic polyurethane and to form an outer sheath of polyvinyl chloride or a polyvinyl chloride/urethane mixture and to provide a web of threads or rovings composed of an aromatic polyamide (i.e., comprised of synthetic threads having a greater tensile strength than optical elements) between both the inner and outer sheath as well as between the inner sheath and a cable core (for example see German OS No. 2,551,211, which approximately corresponds to U.S. Pat. No. 4,076,382, or German AS No. 2,628,069).
Optical communication cables are utilized in various environments, for example in transmitting select light signals from within high voltage systems or spaces which are charged with a pressurized gas, such as in nuclear reactors. In such pressurized environments, optical communication cable structures must be substantially gas-impermeable.